The present invention relates to offset calibration systems for image sensor digitizers, and in particular to black level offset for CCD sensors.
CCD image sensors are used in both motion and still image acquisition applications, such as video camcorders, image scanners and digital still cameras. To get improved image quality, any offset in the black level of the CCD output signal must be corrected.
In U.S. Pat. No. 4,903,144, Stefanik et al. teaches a method which combines digitized data from a reference image and digitized data from the CCD""s optical black (OB) pixels (OB pixels are covered so they receive no light), to generate an offset correction signal. The requirement of a reference image adds to the complexity and cost of the system and requires a special calibration time separate from the active image acquisition time.
In U.S. Pat. No. 5,408,335, Takahashi et al. teaches various servo control loop methods of comparing digitized data from the CCD""s optical black (OB) pixels to a reference value, and then generating a correction signal which is summed with the CCD image signal. These methods may cause undesirable image artifacts if there is noise in the OB pixels. Also, these methods require high resolution digital-to-analog converters (DACs) to create the correction signal, because the correction signal is added to the CCD signal before amplification.
Other patents relating to black level offset include U.S. Pat. No. 5,105,276, DC Restoration of Sampled Imagery Signals; U.S. Pat. No. 5,111,311, Image Reading Apparatus with Black Level Correction; U.S. Pat. No. 5,121,119, Analog-to-Digital Conversion Method and System with Correction of Analog Gain and Offset; U.S. Pat. No. 5,448,306, Image Processing Apparatus with Variable Clamping; and U.S. Pat. No. 5,659,355, CCD Dark Mean Level Correction Circuit Employing Digital Processing and Analog Subtraction Requiring No Advanced Knowledge of Dark Means Level.
The present invention provides an improved offset correction circuit for an image digitizing system having a correlated double sample and hold circuit, a programmable gain amplifier and an analog-to-digital converter. The output of the analog-to-digital converter is provided to a dual offset correction circuit. The dual offset correction circuit provides both first and second correction values as feedback signals.
In one embodiment, the first correction value is a coarse correction which is applied prior to amplification by the programmable gain amplifier. The second correction value is a fine correction offset which is applied as feedback after the programmable gain amplifier.
In one embodiment, the present invention also provides a clipping circuit which limits an input value to the offset correction circuit to a predetermined threshold. This threshold is set to a maximum amount to be expected from an optical black pixel. This thus limits the effective magnitude of any hot pixel in the optical black area. The use of a clipping circuit has been found by the inventors to be simpler and just as effective as identifying and eliminating hot pixels in the optical black area.
In another embodiment of the invention, the offset correction circuit includes an averaging circuit. The averaging circuit computes the average values of data corresponding to optical black pixels. The averaging circuit filters any noise from the digitized optical black pixel values.
In another embodiment of the invention, the offset correction circuit includes a comparison circuit. The comparison circuit compares a measured error value to a reference value. An offset correction is generated only if the reference value is met or exceeded. Such a comparison to a threshold ensures that the system will converge and remain stable, and suppresses the effects of noise.
For a further understanding of the nature and advantages of the invention, reference should be made to the following description taken in conjunction with the accompanying drawings.